Obtaining an Arterial Blood Gas
The first and most important diagnostic study in determining acid base status is the arterial blood gas. Unfortunately, arterial blood gas analysis is prone to various inaccuracies due to error in obtaining the sample and therefore it is imperative that proper sample technique be used.
Pre-sampling preparation
- Before sampling, verify that an order for the arterial blood gas has been written in chart, and that you have identified the right patient for sampling. (Doing an ABG on the wrong person will not give you accurate results, just trust me on this one)
- If time permits, review medical chart; check diagnosis, current respiratory modalities, previous results of arterial blood gases, and current medications. If a recent change has been made in oxygen flow rate, ventilation settings, etc., the practioner should wait at least 20-30 minutes after the change before obtaining the blood gas sample.
- Gather necessary materials. Prior to entering room, you should gather all the material that will be needed to obtain a blood gas sample. The materials needed include the following:
- Gloves, mask
- heparinized blood gas syringe with cap
- a 20 to 22 gauge , short beveled needle
- an alcohol swab
- sterilized 4 by 4 gauze
- small plastic bag with ice
- a label with the patient’s name, hospital number, date, time, and respiratory modality.
- Identify artery for sampling. The most commonly sampled arteries include the radial artery, brachial artery, and femoral artery. Of these three, the radial artery is the preferred sampling site owing to three primary factors: a) it is easy to access, b) it is a superficial artery and therefore easier to palpate, stabilize, and puncture, and c) it has a collateral blood flow. If damage to the radial artery occurs or it becomes obstructed, the ulnar artery will supply blood to the tissues normally supplied by the radial artery. To assess radial artery for sampling, must perform the modified Allen’s test to ensure ulnar artery patency.
- If Allen’s test is negative for both hands and radial artery is not accessible, then the brachial artery may be used. The potential for obtaining a venous sample is greater when using the brachial artery for arterial puncture because there are large veins lying in close proximity to the brachial artery. Additionally, the medial nerve lies parallel to the brachial artery and will cause the patient pain if you inadvertently nick it with a needle.
- The femoral artery is the least preferred arterial sampling site because it is a relatively deep artery; it lies adjacent to the femoral nerve and vein; and it does not have collateral blood flow. Puncture of the femoral artery is usually reserved for emergent situations or for the severely hypotensive patient who has poor peripheral perfusion.
Performing the arterial puncture
- Cleanse the skin at the puncture site with the alcohol swab. The patient’s hand should be bent back slightly, or a small rolled towel placed under the wrist. This brings the radial artery closer to the surface. Overextending the wrist should be avoided as this might occlude the pulse.
- Palpate for the pulse with your index and middle fingers. After locating the strongest pulse sensation, slightly anchor the artery with your index and middle fingers. This will prevent the artery from rolling when you puncture it.
- The syringe should be held at 45° angle or less in your opposite hand, much like you hold a pencil or a dart. This near parallel insertion of the needle will minimize trauma to the artery and allow the smooth muscle fibers to seal the puncture hole after you withdraw the needle.
- While anchoring the artery and with the bevel of the needle turned upward, insert the needle to just under the skin surface. Now advance the needle slowly until you notice a flashing pulsation of blood in the hub of the needle. Stop and maintain this position until you have collected 2 to 4cc of blood in the syringe.
- If you should advance the needle too far, withdraw it slowly until blood flows into the syringe. There should be no need to aspirate the blood into the syringe as arterial pressure will allow auto filling of the syringe. Only in the event that you are using a small gauge needle (e.g. 25 gauge), or the patient is hypotensive, should you aspirate the syringe.
- After obtaining the desired amount of blood, withdraw the needle and apply pressure to the puncture site with the 4×4 gauge. After pressure has been applied for 2 minutes, check the site for bleeding, oozing, or seepage of blood. If present, apply pressure until all bleeding has stooped. A long compression time will be necessary for patients on anticoagulant therapy or who have bleeding disorders.
- Remove the needle from the syringe. Needle should never be recapped, bent, or purposely broken because of the danger of self puncture. All needles should be placed in designated, puncture resistant containers (commonly known as sharps container)
- It is vitally important that air bubbles are removed from the blood gas syringe as they may alter the blood gas results. Hold the syringe upright and gently tap the syringe so that the air bubbles are forced to the top of the syringe where they can be expelled.
- Cap the syringe and place the syringe in the bag of ice (icing the sample will prevent any further metabolism of blood). Attach the lab slip to the bag, and have the sample transported to the laboratory. If you are going to analyze the sample, you should do so as soon as possible.